If doctors say that 99-99.4 temp is probably my normal body temp, then how come at times it comes back to 98.6 or evn less? shouldnt body vary less?

Body temp: No temperature reading alone can determine normal from abnormal. Usually more important, is the patient's history and symptoms. A person with a temp of 99.4 and no symtpoms has nothing to worry about, versus the patient with the same temp and additional symptoms. Also keep in mind that temp readings can vary from on thermometer to another and can also vary dependning on the time of day.